Optical amplifiers are e.g. employed in the field of optical transmission technology for amplifying optical signals transmitted in optical networks. The optical signals in many cases propagate over hundreds of kilometers in an optical fiber and are consequently attenuated during propagation. Using fiber amplifiers, such as erbium-doped fiber amplifiers (EDFAs), the light signals can be amplified in the optical domain, i.e. without intermittent conversion to electrical signals to maintain sufficient signal strengths along the link in spite of the long transmission distance. In this way, a sufficient signal-to-noise ratio at the receivers is achieved.
However, particularly wavelength division multiplexing (WDM) networks suffer from sudden changes in optical power due to component failures, fiber breaks or protection switching. Another source of optical variations is the adding and dropping of optical channels in the WDM network for the purpose of routing of optical signals to their destination. Due to non-linear fiber effects and the non-ideal dynamic properties of fiber amplifiers, such as EDFAs, these changes can propagate to other sites leading to optical power fluctuation across the whole network and possibly to oscillations. Consequently, even wavelength channels that are not directly affected by the switching operations or failures can suffer from some performance degradation at the receivers. Such performance degradation is mainly due to the deviation from the dynamic range of the optical receivers, signal distortions induced by non-linear effects in the transmission fibers, and deterioration of the signal-to-noise ratio.
Furthermore, gain variations can also accumulate in a cascade of amplifiers. Thus, even small gain variations can result in significant power changes at the receivers. Consequently, efficient amplifier control techniques are required that allow to keep the inversion in the gain medium and as a consequence the gain profile of the amplifier or an amplifier stage relatively constant even if the input power changes.
Fast electronic control architectures are currently the most economical solution to stabilize the gain of EDFAs. Herein, typically feed-forward and feedback control techniques are combined. A feed-forward control allows for reacting quickly to input power changes and prevents large gain deviations, but some permanent variations are unavoidable due to inaccuracies of the underlying models, aging effects and intrinsic effects. These deficiencies are compensated by the much slower feedback system cleaning up for any error in the predetermined adjustment made in the feed-forward control and thus helping to recover the original gain of the amplifier, i.e. the gain of the amplifier prior to the sudden change in input power. This combination allows making the feed-forward control robust against aging effects and changing environmental conditions by continuously updating the control parameters during operation. On an again larger time scale, corrections can be made by the link control including the continuously running signal preemphasis.
In a standard control scheme, where a change in the input power to the amplifier is detected, the feed-forward control usually sets the pump power immediately to a new power level. In combination with the altered signal powers, this new pump power level is usually intended to lead to the same steady-state inversion level of the gain medium existing before the input power change. In order to achieve optimum results, the prediction of the new pump power level should be as accurate as possible and almost immediate. But in some cases, even this is not sufficient.
Even in the ideal case of immediate pump power adaption and accurate prediction of the required pump power, gain variations can usually not be avoided completely. This is particularly true for EDFAs which use at least one pump with an emission wavelength of around 980 nm. With this pump wavelength, the Er3+ laser active dopant will be pumped from the gound state 4I15/2 to the “third state” 4I11/2. A relatively fast multi-phonon transition leads from the third state 4I11/2 to the metastable state 4I13/2, which has a lifetime depending on the glass composition on the order of 8 ms to 15 ms with a typical value of 10 ms. Due to the finite lifetime at the third state or energy level, transitions from the third energy level to the metastable state or second energy level are not adapted synchronously to a sudden reduction of the pump power due to the finite lifetime of this level, since, metaphorically speaking, some of the pump photons are stored at the third level. This gives rise to a memory effect that leads to a delayed reaction to control interactions causing intermediate gain variations. Such intermediate gain variations can in fact be large enough to strongly disturb data detection at the receivers. Due to the accumulation of gain variations in optical networks, as mentioned before, there is a strong need to keep gain variations as small as possible.
One way of suppressing transient dynamic gain fluctuations referred to as “input delay control” is disclosed in H Nakaji, Y Nakai, M Shigematsu, and M Nishimura, “Superior high-speed automatic gain controlled erbium-doped fiber amplifiers,” Opt. Fiber Techn., vol. 9, no. 1, pp. 25-35, February 2003. Herein, it is proposed to delay the signal between an input monitor and the actual input to the first EDF of the amplifier in order to be in a position to adjust the pump power before the change of the input power to the gain medium actually occurs. However, the delay as proposed by Nakaji et al. requires an additional fiber having a length of at least 400 m. In addition to a small increase in noise, this additional fiber also requires larger amplifier housings for accommodating the fiber coil, contrary to the current trend towards smaller amplifier sizes.
An improved method of optical fiber amplifier control without any input delay is disclosed in EP 2 320 582 A1 by the present inventor. In this control method, when a power drop at the input of the amplifier is detected, the pump is switched off completely (or at least reduced to a power level close to zero) during a limited time period referred to as “zero period” in the present disclosure, in order to depopulate the third level of the Er3+ ions more quickly.
While this “zero period technique” leads to a significant reduction of transient gain variations as a consequence of input power variations, still depending on circumstances, a need for even lower gain variation may arise. This is particularly true in cases where pump bypasses or pump splitters are employed for introducing pump power into two different rare earth doped fiber coils separated by an optical isolator.